On a hot August day in 2005 my team and I were out hunting for fish fossils in the tall, grassy paddocks of Gogo Station, a vast cattle ranch located in the heart of northwestern Australia. Today the arid region is hardly suitable for aquatic creatures. But some 375 million years ago, during the Late Devonian period, a shallow sea covered the area and Gogo was home to an enormous tropical reef that teemed with marine life, including a plethora of primitive fishes. Luckily, many of their remains have survived across the ages. Nestled among the clumps of spiky Spinifex bushes and sleepy death adders lie softball-size nodules of limestone—the products of millions of years of erosion of the local shales—some of which harbor pristine fossils of the fishes that lived on the primeval reef. And so over the course of our terrestrial fishing expedition, we would spend our days cracking the nodules open, one after another, hoping to glimpse a treasure inside.

The most abundant of the fishes that patrolled the Gogo reef were armored creatures called placoderms (“plated skin”)—some of the first backboned animals with jaws. Though gone today, placoderms ruled the planet for nearly 70 million years, making them the most successful vertebrate group of their time. Scientists have long debated exactly how they are related to other backboned creatures, and we had come to Gogo to look for specimens that might help resolve this and other questions about fish evolution. On this particular day our efforts were rewarded with a nodule containing what appeared to be a fairly complete fish. It did not strike me as especially remarkable in its anatomy, though, just another placoderm fossil to add to our haul and take back to the lab for extraction from its limestone tomb at a later date. Little did I know that this seemingly modest find would upend scientists’ understanding of a very intimate aspect of vertebrate biology—the origin of sexual intercourse and internal fertilization.